The Beach-hopper Genus Platorchestia (Crustacea: Amphipoda: Talitridae) on Atlantic Ocean Coasts and on those of Associated Seas Author Myers, Alan A. School of Biological, Earth and Environmental Sciences, University College Cork, Cork Enterprise Centre, Distillery Fields, North Mall, Cork, Ireland Author Lowry, James K. Australian Museum Research Institute, 1 William Street, Sydney NSW 2010, Australia (deceased 4 November 2021) text Records of the Australian Museum 2023 Rec. Aust. Mus. 2023-12-06 75 4 485 505 http://dx.doi.org/10.3853/j.2201-4349.75.2023.1887 journal article 293309 10.3853/j.2201-4349.75.2023.1887 b7c830ce-020a-4281-bf79-7f344fa5acb0 2201-4349 10413693 Platorchestia Bousfield, 1982 Platorchestia Bousfield, 1982: 26 . Included species . Platorchestia includes 11 established species: P. ano Lowry & Bopiah, 2013 ; P. exter sp. nov. ; P. griffithsi sp. nov. ; P. munmui Jo, 1988 ; P. negevensis sp. nov. ; P. oliveirae sp. nov. ; P. pachypus ( Derzhavin, 1937 ) ; P. pacifica Miyamoto & Morino, 2004 ; P. paraplatensis Serejo & Lowry, 2008 ; P. platensis ( Krøyer, 1845 ) — type species by original designation; P. smithi Lowry, 2012 and one putative species, the incompletely described P. crassicornis ( Costa, 1867 ) . Diagnosis . Antenna 1 short, not longer than article 4 of antenna 2. Antenna 2 peduncle article 3 without ventral plate; articles 4–5 generally incrassate in males. Maxilliped palp article 2 with distomedial lobe; article 4 reduced, button-shaped. Gnathopod 1 sexually dimorphic; subchelate, cuspidactylate. Gnathopod 2 subchelate in males, mitten-shaped in females. Pereopods 3–7 cuspidactylate. Pereopod 7 often incrassate in terminal males. Uropod 1 endopod without marginal setae. Telson with apical and marginal robust setae. Remarks . Beach hoppers of the genus Platorchestia live amongst algal debris, high on marine shores sometimes in estuaries and among mangroves. One Atlantic species has become riparian. In males, pereopod 7 is generally sexually dimorphic, being more robust or incrassate in males—the only exception to this among Atlantic species is P. negevensis sp. nov. , although it is a frequent state elsewhere. In particularly large males (herein referred to as hyperadults), the carpus of pereopod 7 becomes markedly incrassate, either subrectangular or subovoid and sometimes the anterior margin may be crenulate or notched. Hyperadult males may be quite uncommon in a population, so that large samples of a population may be collected including relatively large males, none of which exhibit full incrassation of pereopod 7. Nevertheless, the type of incrassation found in hyperadult males is of specific importance. When hyperadult males are not represented in a collection, other character states must be observed for correct identification.